Mrs Bourne, 60, who is married to Freddie's father Edward, wrote: "It is high time someone explained to you about good manners. Yours are obvious by their absence and I feel sorry for you."
She went on: "Unfortunately for Freddie, he has fallen in love with you and Freddie being Freddie, I gather it is not easy to reason with him or yet encourage him to consider how he might be able to help you. It may just be possible to get through to you though. I do hope so. Your behaviour on your visit to Devon during April was staggering in its uncouthness and lack of grace.
"Unfortunately, this was not the first example of bad manners I have experienced from you. If you want to be accepted by the wider Bourne family I suggest you take some guidance from experts with utmost haste."
She even described her future daughter-in-law as "an ideal candidate for the Ladette to Lady television series".
Mrs Bourne, who lives near Dawlish in Devon and is a renowned breeder of pinks and dianthus flowers, went on to list numerous examples of Miss Withers's "lack of manners" - as detailed below.
She concluded by saying: "I pity Freddie."

GENEVA (AFP) – Ten million people in the Horn of Africa have been hit by the worst drought in 60 years, with some areas on the verge of famine and thousands on the march in search of food and water, the UN said on Tuesday.
A poor rainy season coupled with rising food prices have led to severe food shortages in countries including Djibouti, Ethiopia, Kenya, Somalia and Uganda.
Cattle and sheep are dying at higher rates than usual, reaching up to 60 percent of mortality in some areas.
"Over 10 million people are affected by the drought in one way or other," said Elisabeth Byrs, spokeswoman for the UN Office for the Coordination of Humanitarian Affairs.
"We believe that the drought situation in certain regions is the worst in 60 years," she said.